There is an ongoing discussion about whether the hotspot is caused by a deep
mantle plume or originates at a much shallower depth. Foulger et al. believe the Icelandic plume reaches only to the mantle transition layer and can therefore not come from the same source as Hawaii. Bijwaard and Spakman, however, believe the Icelandic plume does reach to the mantle, and therefore comes from the same source as Hawaii. While the Hawaiian island chain and the
Emperor Seamounts show a clear time-progressive volcanic track caused by the movement of the
Pacific Plate over the Hawaiian hotspot, no such track can be seen at Iceland. It is proposed that the line from
Grímsvötn volcano to
Surtsey shows the movement of the
Eurasian Plate, and the line from Grímsvötn volcano to the
Reykjanes volcanic belt shows the movement of the North American Plate.
Mantle plume theory The
Iceland plume is a postulated upwelling of anomalously hot rock in the Earth's
mantle beneath
Iceland. Its origin is thought to lie deep in the mantle, perhaps at the
core–mantle boundary at about depth. Opinions differ as to whether seismic studies have imaged such a structure. In this framework, the volcanism of Iceland is attributed to this plume, according to the theory of
W. Jason Morgan. It is believed that a
mantle plume underlies Iceland, of which the hotspot is thought to be the surface expression, and that the presence of the plume enhances the volcanism already caused by plate separation. Additionally, flood basalts on the
continental margins of
Greenland and
Norway, the oblique orientation of the
Reykjanes Ridge segments to their spreading direction, and the enhanced igneous crustal thickness found along the southern
Aegir and
Kolbeinsey ridges may be results of interaction between the plume and the
Mid-Atlantic Ridge. The plume stem is believed to be quite narrow, perhaps across and extending down to at least beneath the Earth's surface, and possibly down to the
core-mantle boundary, while the plume head may be greater than in diameter. It is suggested that the lack of a time-progressive track of seamounts is due to the location of the plume beneath the thick Greenland
craton (Laurentia) for ~ 15 after continental breakup, and the later entrenchment of the plume material into the northern Mid-Atlantic Ridge following its formation. This coincides with the opening of the north Atlantic in the late
Paleocene and early
Eocene, which has led to suggestions that the arrival of the plume was linked to, and has perhaps contributed to, the breakup of
Laurasia. In the framework of the plume hypothesis, the volcanism was caused by the flow of hot plume material initially beneath thick continental lithosphere and then beneath the lithosphere of the growing ocean basin as rifting proceeded. The exact position of the plume at that time is a matter of disagreement between scientists, as is whether the plume is thought to have ascended from the deep mantle only at that time or whether it is much older and also responsible for the old volcanism in northern Greenland, on
Ellesmere Island, and at
Alpha Ridge in the Arctic. As the northern Atlantic opened to the east of Greenland during the Eocene, North America and Eurasia drifted apart; the
Mid-Atlantic Ridge formed as an oceanic spreading center and a part of the submarine volcanic system of
mid-oceanic ridges. The initial plume head may have been several thousand kilometers in diameter, and it erupted volcanic rocks on both sides of the present ocean basin to produce the
North Atlantic Igneous Province. Some geologists have suggested that the Iceland plume could have been responsible for the
Paleogene uplift of the
Scandinavian Mountains by producing changes in the density of the
lithosphere and
asthenosphere during the opening of the North Atlantic. To the south the Paleogene uplift of the English chalklands that resulted in the formation of the
Sub-Paleogene surface has also been attributed to the Iceland plume. An extinct ridge exists in western Iceland, leading to the theory that the plume has shifted east with time. The oldest crust of Iceland is more than 20 million years old and was formed at an old oceanic spreading center in the
Westfjords (Vestfirðir) region. The reorganisation of the plate boundaries in Iceland has also been attributed to microplate tectonics,
Subducted ocean plate According to one of those models, a large chunk of the subducted plate of a former ocean has survived in the uppermost mantle for several hundred million years, and its oceanic crust now causes excessive melt generation and the observed volcanism. This convection mechanism is probably not strong enough under the conditions prevailing in the north Atlantic, with respect to the spreading rate, and it does not offer a simple explanation for the observed geoid anomaly. == Geophysical and geochemical observations ==